This invention generally relates to asset tracking systems and, more particularly, to a method for lessening time error in a reduced-order Global Positioning System (GPS) localization system (i.e. one that does not calculate an accurate location at a tracked object) without requiring use of a highly accurate time clock.
Twenty-one Navstar GPS satellites in medium-altitude earth orbits make up the GPS satellite constellation. Signals transmitted from these satellites allow a receiver near the ground to accurately determine time and its own position. Each satellite transmits data that provide precise knowledge of the satellite position and allow measurement of the distance from that satellite to the antenna of the user's receiver. With this information from at least four GPS satellites, the user can compute its own position, velocity and time parameters through known triangular techniques (i.e. the navigation solution). Typically, seven, but a minimum of four, satellites are observable by a user anywhere on or near the surface of the earth if the antenna of the receiver operated by the user has an unobstructed view of the sky, down to very near the horizon. Each satellite transmits signals on two frequencies known as L1 (1.575.42 MHZ) and L2 (1277.6 MHZ) and all satellites share these frequencies using CDMA (code division multiple access) DSSS (direct sequence spread spectrum).
Each satellite transmits a single high-resolution DSS signal on frequency L2 and the same signal plus another lower-resolution DSSS signal on frequency L1. The low-resolution DSSS signal comprises a P/N (pseudo-random noise) code with a 1.023 MHZ chipping rate and a 1.0 ms repetition period, and a message data sequence (NAV data) with a rate of 50 bits per second. The high-resolution DSSS signal uses a P/N code with a 10.23 MHZ chipping rate and a repetition period longer than a week. The same NAV data stream is used in all DSSS signals from a given satellite. The NAV message from a given satellite contains the GPS signal transmission time, ephemeris (position) data for that satellite, almanac data (a reduced accuracy ephemeris) for all of the satellites in the constellation, and a hand-over word for use in connection with the transition from low-resolution to high-resolution code tracking. The low and high-resolution codes are known as the course/acquisition (C/A) and precise (P) codes, respectively.
After acquisition, the offset of each code, together with the signal-transmission time from the NAV data, enables a receiver to determine the range between the corresponding satellite and the user. By including both the P code and the repeating C/A code in the transmitted signal, a more-rapid hierarchical acquisition of the P code is made possible and a two tiered level of global navigation service can be provided. The P code can provide positions that are accurate to approximately 3 meters, while the C/A code yields accuracies on the order of 30 meters. Typically, the low-resolution service is unrestricted while the high-resolution service is restricted to the military by encrypting or otherwise controlling knowledge of the high-resolution P/N code.
Received GPS signals are usually shifted in frequency from the nominal L1 and L2 carrier frequencies because the GPS satellites move in orbit at several kilometers per second, yielding a substantial Doppler shift. The satellite trajectories are usually known a priori and the Doppler shifted carrier frequencies are therefore predictable if the GPS receiver location is known. However, the receiver location is not known a priori, and there is often substantial local oscillator error with inexpensive receivers. The resulting uncertainty in received carrier frequency (i.e., in needed replica carrier frequency) can be large (e.g., .+-.7.5 kHz), and this frequency range may have to be searched during the GPS signal-acquisition process. The frequency or Doppler search is usually done by repeating the cross-correlation of the received sample and local replica P/N sequences for different local oscillator (carrier replica) frequencies. The spacing between frequency steps is made small enough to avoid missing the signal when long cross-correlation integration times (narrow filter bandwidths) are used. Long integration times improve detection of low SNR (signal-to-noise ratio) signals. With typical civilian GPS applications, 1.0 millisecond cross-correlation integrations are used (a single C/A code cycle), yielding an equivalent Doppler filter bandwidth of approximately 500 Hz. A .+-.7.5 kHz frequency range can be searched with thirty 500 Hz steps. The GPS acquisition then entails a search over satellite code, code offset, and Doppler frequency.
A master control station (MCS) and a number of monitor stations comprise the control portion of the GPS. The monitor stations passively track all GPS satellite in view, collecting ranging data and satellite clock data from each satellite. This information is passed to the MCS where the future ephermeris and clock drift are predicted for the satellites. Updated ephermeris and clock data are uploaded to each satellite for re-transmission in each NAV message from each satellite.
In operation, a typical GPS receiver performs the following for each of at least four satellite signals:
1) acquires the DSSS signal PA1 2) synchronizes with the NAV data steam and reads the satellite time-stamp, clock-correction, ionospheric-delay and ephermeris data, PA1 3) calculates the satellite position from the ephemeris data, PA1 4) reads its own receiver clock to determine the receiver time associated with reception of the time-stamp epoch, and PA1 5) estimates the signal travel time by subtracting the time-stamp value from the associated receiver time.
This time difference is multiplied by the speed of light to obtain an estimated range to the satellite. If the GPS receiver clock were perfectly synchronized with the clocks of the satellites (or the error were known), only three such range estimates would be required to precisely locate the receiver. There is, however, a clock-bias (slowly changing error) due the fact that the satellites are equipped with atomic clocks while the GEPS receivers typically use less accurate locks. This clock bias is learned and its effect eliminated by measuring the range (travel time) from four GPS satellites and using these measurements in a system of four equations with four unknowns (receiver x, y and z, and time).
In an application of the invention, assets such as railcars, shipping or cargo containers, trucks, truck trailers, and the like are located and tracked by using the GPS. In asset tracking, the GPS receivers are usually battery powered since an independent source of power is generally not available. It is advantageous to increase the operating life of the batteries by reducing the energy consumed by the GPS receiver.
In a typical GPS receiver, the receiver front end (i.e. radio frequency, (RF) and intermediate frequency (IF) electronics) consumes a large amount of power while it is turned on. This results in high energy consumption if the signal acquisition and synchronization are not quickly accomplished. Most GPS receivers do not have signal storage (memory) and must process the received signals in real time. Such receivers use either a sequential search or search a small number of satellite/code-offset/Doppler (SCD) bins simultaneously to achieve signal acquisition. These receivers must continually receive and process each satellite signal until the SCD bin for that signal is identified and the necessary NAV data are decoded.
In a system where a central facility or station must keep track of multiple assets (e.g. railcars), each tracked object may carry a GPS receiver that processes data from several of the visible GPS satellites; often, however, an accurate position determination is not made at the receiver. Instead, only partial processing is done at the receiver and intermediate results are transmitted from the asset to the central station. These intermediate results do not necessarily require decoding of navigational or other data from the GPS signals, and thus may allow the GPS receiver and signal processor to be powered only long enough to acquire the satellite signals (i.e. determine the SCD bins). In such system, the dominant energy consumption occurs in the acquisition process, and the GPS receiver energy expended at each tracked asset can be dramatically reduced if the signal acquisition time and energy are dramatically reduced. To accomplish this result, however, an accurate (and hence, expensive) time clock would be needed at the asset in order to avoid substantial location errors. It would thus be desirable to provide a method for reducing time error at the tracked GPS receiver in a reduced-order GPS localization system while using a drift-susceptible time clock.